In wireless communication, the demand for high data rate keeps increasing and Long Term Evolution (LTE) developed by the third generation project partnership (3GPP) has been approved an extremely successful platform to meet such demand. The LTE system has been designed to operate in dedicated and licensed band to avoid interference with other systems and to guarantee satisfying communication performance. However, with demand for high data rate keeps increasing and at the same time available licensed frequency resources keeps shrinking, more and more cellular network operators begin to consider utilization of the unlicensed spectrum as a complimentary tool to augment their service offering.
One alternative way for utilizing an unlicensed band is called “Licensed-Assisted Access (LAA)”, which means the utilization of the unlicensed band is under control from the licensed band. LTE LAA is a topic to be studied in 3GPP LTE-Advanced Rel-13 and beyond. The objective of LTE LAA is to investigate the basic aspects for the operator-controlled non-standalone deployment of LTE in unlicensed spectrum, considering uplink and downlink or downlink only transmission in the unlicensed spectrum, to further improve network throughput and provide offloading capability to meet the demand of increasing traffic volume.
Particularly, LTE LAA can use carrier aggregation (CA) to aggregate the carriers in unlicensed spectrum, i.e., using unlicensed carrier as a Supplemental Downlink or a Component Carrier. In such a scenario, a primary cell (also called PCell, primary carrier, or primary component carrier) for either a LTE frequency division duplex (FDD) or time division duplex (TDD) system can always operate in a licensed band to carry control signaling, mobility management and data, while one or more secondary cells (also called SCells, secondary carriers, or secondary component carriers herein) in unlicensed band can provide downlink (DL) and/or uplink (UL) data transmission for opportunistic capacity improvement.